Thursday, October 28, 2010

"Category Five": How a Hurricane Yardstick Came to Be

"It catches their attention and inspires them to take more action to prepare for a storm than if there was no scale at all," he added. "It's been really good in the respect of having the public be aware of strong storms and take more drastic measures to protect themselves."

Simpson and Saffir each witnessed powerful hurricanes long before they devised the scale. Simpson watched a hurricane deliver a devastating blow to his hometown of Corpus Christi, Texas, in 1919. Although he was only six years old at the time, he still has vivid memories of that catastrophe.

In the summer of 1947, Saffir, a New York City native, took a job as a county engineer in Miami. Soon after he arrived, a pair of hurricanes put much of South Florida under water.

About 12 years later, Saffir had set up his own engineering firm when the United Nations commissioned him to write a report on providing low-cost housing in areas of the world that are subject to hurricanes.

As he worked on the report, Saffir realized that there were only two ways of categorizing hurricanesmajor and minor.

"I worked up five scales [of hurricanes] based on a subjective description of the damage that could occur for each category," Saffir said.

Soon after Saffir's report to the UN, Hurricane Camillethe second-most powerful hurricane in U.S. historystruck in August 1969. The storm blasted coastal Mississippi with a 25-foot (7.6-meter) storm surge and winds exceeding 190 miles an hour (300 kilometers an hour).

The hurricane's torrential rains also unleashed deadly flooding far inland as the storm moved across the southeastern U.S.

"Hurricane Camille was sort of a red light to the federal government to learn what can happen with a hurricane," Saffir said. "Before Camille, I think most people were thinking of a hurricane as a sort of local event. Camille was a warning."

Alert System

In Camille's wake, federal officials decided a system was needed to alert local officials and relief agencies of what to expect when a hurricane is predicted to make landfall.

"We had so many agenciesthe Red Cross, the Salvation Armythat didn't understand the characteristics of storms enough to make decisions about how they needed to plan their responses," Simpson said. "We could tell them the location where the storm would strike, but not what they needed to send."

Saffir conferred with Simpson, who had become director of the National Hurricane Center in 1967. Simpson came up with the idea to refine Saffir's five categories by including a hurricane's lowest barometric pressure readings and likely storm surge levels in the ranking system.

The barometric pressure reading in the eye of a hurricane drops as the storm intensifies. Extremely powerful hurricanes have very low barometric pressure readings in their centers.

For example, in October 2005 Hurricane Wilma's barometric pressure dropped to 26.04 inches, or 882 millibarsthe lowest ever recorded for a hurricane in the Atlantic Ocean.

At sea level during calm weather, the normal barometric pressure is roughly 30.00 inches, or about a thousand millibars.

"The bottom line is that central pressure is the most conservative thing that you have in a hurricane," Simpson said. "It's the best yardstick as to what the potential damage is going to be. The central maximum wind may be going up or down, all within the same category of hurricane."

The National Hurricane Center started using the Saffir-Simpson scale experimentally in 1970. At first, the scale was made available only to meteorologists and relief agencies. Then the federal government decided that it should be released to the general public.

Irene Toner, emergency management director of Monroe County, Florida, is one of hundreds of public officials who now routinely use the Saffir-Simpson scale to prepare for hurricanes. But Toner, whose district includes the Florida Keys, adds an extra twist to hurricane preparation.

"If they tell me a Category One is coming, I'll prepare for a Category Two," she said.

Willie Drye is the author of Storm of the Century: The Labor Day hurricane of 1935, published by National Geographic Books.